The Two Diversifications of Animal Phyla

With the advent of this form—what evolutionary biologists call the "roundish flatworm"—a body plan was in place that could be modified to shape all the categories of metazoan life, the major body plans that we call phyla. The phyla living today include the arthropods; the mollusks; the echinoderms; our own group, the chordates; and about 25 more. These are the complex metazoans that we hope to find—but that may be very rare—on other planets. These are animals. They appeared relatively late in the history of life on Earth. One of the great novel insights of the 1990s was our realizing that their origin and their subsequent diversification and rise to abundance were two separate events, not one, as had been believed since the time of Charles Darwin.

Fossils of macroscopic animals (those visible to the unaided eye) first appear in abundance less than 600 million years ago, during the "Cambrian Explosion," a diversification event resulting in the rapid formation of thousands of new species; we will describe it in more detail in the next chapter. Yet the appearance of abundant animal fossils at this time actually marks the second of the two diversification events that led to the proliferation of larger animals on the planet. As we will show, fossils of such complex animals as trilobites and mollusks—common members of the Cambrian Explosion—are advanced descendants of a much earlier, diversification event that took place between 1 billion and 600 million years ago. Yet there is no fossil record of this first diversification—paleontologists have been stymied by an almost compete lack of fossils in strata older than 600 million years, when this initial event must have taken place. Our understanding of the initial diversification of animals comes not from paleontology but from an entirely different line of investigation: genetics. Geneticists have arrived at answers about the "when" of the first diversification event by examining the genetic code of living animals via a technique called ribosomal RNA analysis.

Gene sequences are simply strings of base pairs lined up along the double helix of a DNA molecule. As we saw earlier, if a DNA molecule is likened to a twisted ladder, the base pairs can be considered the steps of the ladder, and it is the sequence of the steps that is used in this type of analysis. Genes are simply instructions for protein formation coded by the sequence of nu-cleotides on the DNA ladder. There are only four types of nucleotides, but they provide the genetic code that is the basis for all Earth life. All organisms share more genes with their ancestors than with nonrelated species. By comparing the genes from various organisms, it is possible to produce a model of evolutionary history (an evolutionary tree, as it were) with the branches of the tree showing which species gave rise to which other species. Yet according to many geneticists, such an analysis not only tells us how the branching occurred, it can also tell us when.

In 1996 G. Wray, J. Levinton, and L. Shapiro published a paper claiming, on the basis of results obtained by using this genetic technique, that the first event—the earliest divergence of animals—occurred 1.2 billion years ago. This result drew a collective gasp from the paleontological fraternity: It seemed much too ancient. The fundamental assumption of the Wray et al. paper is that gene sequences evolve with sufficient regularity that a sort of molecular "clock" can be used to date the divergence of various groups. The reasoning behind the molecular clock technique is that changes in the genetic code—evolution, in other words—occur at a rather constant rate. The more distinct two DNA sequences are, the longer it has been since they diverged from a common ancestor. Other scientists, however, dispute that changes in gene frequency occur at a constant rate, and therefore they do not believe in the molecular clock. It is these molecular clock data that led the Wray group to their conclusion. This finding was a bombshell. If animals evolved this early, why did they not appear in the fossil record until less than 600 million years ago? What were they doing for such a long time?

The Wray group's findings were extremely controversial not only because they contradicted long-held paleontological dogma but also because they provoked criticism among other geneticists. There is fierce debate among geneticists about the reliability of the molecular clock technique. The Wray study itself, yielded both minimum and maximum figures for the earliest divergence. One group of genes suggested that the fundamental splitting of the phylum made up of annelids (worms) from the phylum of chordates (our phylum) occurred only 773 million years ago, whereas a second group of genes (in the same organisms) suggested 1621 million years ago—a very wide spread indeed! These results give us minimum and maximum ages for the di vergence. Even with the minimum figure, however, there were (according to the molecular data, anyway) recognizable chordates and annelids 700 million years ago—yet there is no trace of their presence in the fossil record. Where were they? Or were they not there at all? Could it be that no rocks of this age survive or that no fossils from the interval of about 1 billion to less than 600 million years ago were preserved? This seems to be stretching things, as suggested by British paleontologist Simon Conway Morris:

Appeals to gaps in the rock record and pervasive metamorphism of the sediments are not going to work: if there were large meta-zoans capable of either fossilization or leaving traces, they had an uncanny knack of avoiding areas of high preservation potential.

Since the original, tantalizing analysis by the Wray group, other geneticists have reconsidered the basic data. Most concede that the 1.2-billion-year figure is too old. (However, a report published in Science magazine in late 1998 by a team headed by Adolf Seilacher of Yale University announced the discovery of billion-year-old trace fossils (worm-tracks) possibly derived from small, worm-like organisms. Critics of this finding suggest that the marks in question could just as easily have been produced by inorganic actions, and even if these trace fossils turn out to have been produced by organisms, the question remains: Why are no further such fossils found for hundreds of millions of years?) Let's say, then, that divergence occurred less than a billion years ago. We must still account for a significant period of time with animals but without fossils. Paleontologists have long believed that only a single major diversification event occurred—the event coincident with the appearance of fossils, the so-called Cambrian Explosion that began about 550 million years ago. Now this evolutionary event is seen as a follow-up to the much earlier first event.

The answer to this seeming conundrum is that the animals were indeed present, but they were so small as to be essentially invisible in the fossil record. A recent and spectacular discovery of microscopic fossil animal embryos seems to confirm this view. Using newly developed techniques of searching for tiny (but complex) animals in minerals called phosphates, paleontologist

Andy Knoll and his colleagues have uncovered a suite of tiny but beautifully preserved fossils interpreted to be the embryos of 570-million-year-old triploblasts—animals with three body layers, like most of those found today. These fossils tell us that the ancestors of the modern phyla were indeed present at least 50 million years before we find any conventional fossil record of them. The combination of genetic information and new discoveries from the fossil record now give us a robust view of the rise of animals: They did not exist 1 billion years ago, and perhaps not 750 million years ago. Animals are indeed very late arrivals on the stage of life on Earth.

Thanks to these new discoveries and interpretations, the question of "when" has been answered to most people's satisfaction: The emergence of animals was a two-stage event. The initial stage seems to have occurred less (and perhaps much less) than the billion years ago proposed by Wray and his colleagues. But even recalibrated, the Wray group's finding has given us yet another tantalizing insight into the potential incidence of animal life in the Universe. The Wray work confirms that there were indeed two "explosions." The first was the actual differentiation of the various body plans; the second was the differentiation and evolution, in these various phyla, of species large and abundant enough to enter the fossil record. The geneticists can show that genes of annelid worms and genes of chordates were differentiating hundreds of millions of years before the emergence of these creatures as large entities that could appear in the fossil record. This leads us to ask a crucial question: Even if they evolve, do animals necessarily, or inherently, go on to diversify, enlarge, and survive? Does the second flowering of animal life—the Cambrian Explosion event so long known to geologists—inevitably follow the first diversification, or is it yet another threshold of possibility that may be (but is not necessarily) attained? Perhaps on some worlds in the Universe, animals diversify but never attain larger size and greater numbers in some Cambrian Explosion equivalent. This particular insight was first expressed by paleontologist Simon Conway Morris:

We need to discuss to what extent metazoan history was implicit a billion years ago, at least in outline, as opposed to what was inevitable 500 million years later at the onset of the Cambrian ex plosion. Even if metazoans have a deep history, which paleonto-logically remains cryptic, the actual organisms would have been of millimeter size and perhaps without the potential for macroscopic size and complex ecology. . . . Wray et al. may have been correct in tracing the gunpowder back as far into the mists of the Neo-proterozoic (the late Precambrian time period of a billion years ago), but the keg itself still looks as if it blew up in the Cambrian.

In other words, it seems that the development of animals was a two-step process, with step two—the Cambrian Explosion—not necessarily being an outcome predetermined by the initial differentiation of the animal phyla.

Over and over the same question arises: Why did it take so long for animals to emerge on planet Earth? Was it due to external environmental factors, such as the lack of oxygen for so long in the history of this planet, or to biological factors, such as the absence of key morphological or physiological innovations?